1. Field of the Invention
The present invention relates to supercapacitors, and more particularly to a device to prevent overpressure for a supercapacitor.
2. Description of Related Art
Various electrochemical devices, in particular supercapacitors, produce hydrogen during their operation.
A supercapacitor comprises two electrodes with a high specific surface area, between which a separator is placed, this assembly being placed in a closed chamber. The separator and the electrodes are impregnated with a solution of an ionic compound in a liquid solvent.
The supercapacitor generates gas during operation, which is essentially hydrogen. Buildup of the hydrogen formed in the supercapacitor causes an increase in the internal pressure, which is detrimental to the lifetime of the supercapacitor. An internal overpressure can degrade the supercapacitor by deformation, by opening or by explosion.
Various devices have been proposed in the prior art in order to remedy this problem.
Reversible degassing valves are used particularly in lead batteries, referred to as VRLA. They consist of a polymer membrane, in particular a polyethylene membrane. These membranes are not suitable for supercapacitors because they do not prevent entry of air into the device.
Various supercapacitors, in particular some marketed by the companies Maxwell or Epcos, are designed so that the casing has a weak zone which ruptures when the internal pressure exceeds a given threshold. Although such a device avoids any catastrophic behavior of the capacitor (in particular due to explosion), it nevertheless has the drawback of being irreversible and consequently does not allow the lifetime of the supercapacitor to be increased.
Reversible degassing valves exist on various supercapacitors marketed by the company Nippon-Chemicon. In these supercapacitors, the degassing valve comprises an elastomer seal held under pressure by a washer. The liquid of the electrolyte is propylene carbonate (PC) which is a liquid with low volatility, so as to prevent or at least limit deposition of the salt of the electrolyte in the valve. However, when the electrolyte is a salt in solution in a volatile solvent, for example acetonitrile, the risk of the valve being clogged by the salt increases significantly. In fact, a salt deposit at a valve will irreversibly lead to entry of air and water into the supercapacitor. It is well known that water and oxygen are highly reactive chemical species which rapidly degrade the properties of the electrolyte (and potentially the electrodes) thereby very rapidly leading to the end of life of the supercapacitor (U.S. Pat. No. 6,233,135).
The use of acetonitrile compared with propylene carbonate in a supercapacitor is desirable because an electrolyte in which the solvent is acetonitrile has a conductivity higher than that of an electrolyte in which the liquid solvent is PC. Furthermore, the generation of gas is greater over the course of time in the supercapacitor when the solvent is PC. However, the internal overpressure of a supercapacitor leads to its end of life by deformation, by opening or by explosion. For the same ageing conditions, a supercapacitor operating with an electrolyte based on PC therefore generally exhibits a shorter lifetime than when the electrolyte is based on acetonitrile.
DE-10 2005 033 476 describes a device which uses a polymer membrane with selective permeability. The membrane is a so-called “non-porous” membrane through which a gas can pass by diffusion, and not by direct passage. It is in particular a polymer membrane, in particular an EPDM membrane. The elasticity of such a polymer membrane makes it possible to attenuate strong productions of gas inside the device because the membrane can form a bubble, which increases the surface area for transfer to the outside, for example when an increase in temperature causes an increase in the production rate of the gas. However, polymer membranes do not prevent reverse diffusion of undesirable gases such as oxygen, water vapor, carbon monoxide and dioxide, nitrogen oxides or any other gas which is sufficiently small but detrimental to the ageing of supercapacitors which operate in an organic medium or in an aqueous medium.
Numerous metals exhibit permeability to hydrogen. When a membrane consisting of such a metal is placed in a gas flow containing hydrogen, the hydrogen gas dissociates in contact with the membrane's face exposed to the gas flow, the dissociated hydrogen diffuses through the membrane and recombines when it reaches the opposite face of the membrane, and molecular hydrogen escapes from the membrane.
Information relating to the selective permeability of various metals and metal alloys in relation to hydrogen and its isotopes can be found in the literature. In particular, mention may be made of “Review of Hydrogen Isotope Permeability Through Materials”, by S. A. Steward, Lawrence Livermore National Laboratory, University of California, 15 Aug. 1983, which gives data associated with metals and metal alloys, in particular those in the table below.
Φ25° C.Φ70° C.Φ0(mol · m−1 ·Metal(mol · m−1 · s−1 · Pa−1/2)EΦ (K)s−1 · Pa−1/2)Aluminum†  3 10−5148008.1 10−275.5 10−24Copper8.4 10−792902.4 10−201.4 10−18Stainless  1 10−780002.2 10−197.4 10−18SteelNickel3.9 10−766009.4 10−171.7 10−15Palladium2.2 10−718853.9 10−109.0 10−10†Average value, depending on the surface quality;EΦ max = 18900 K.
U.S. Pat. No. 3,350,846 describes a method of recovering hydrogen by permeation through metallic membranes which allow selective diffusion of H2. The membranes consist of Pd, a PdAg alloy, or alternatively they comprise a layer of a group VB metal (V, Ta, Nb) coated on each of its faces with a continuous non-porous film of Pd or an alloy of PdAg, PdAu or PdB. In a preferred embodiment, the membranes are heated to a temperature of between 300° C. and 700° C., a temperature range which is incompatible with an application of the supercapacitor type.
The site http://www.ceth.fr/sepmemfr.php describes a method of purifying a gas using a metallic membrane allowing hydrogen to be separated selectively from a gas mixture. The membrane is an all-metal composite membrane consisting of three layers. A very fine but dense layer of palladium constitutes the active part providing the selective permeability. It is supported by a thin metallic intermediate layer with fine pores, which makes it possible to ensure a very good holding of the dense palladium layer even at high levels of temperature or pressure. The intermediate layer is itself supported by a thicker porous metallic substrate. The hydrogen molecules which arrive in contact with the palladium layer are adsorbed and dissociated, and the elements resulting from the disassociation diffuse through the palladium layer and recombine when they desorb from the palladium.
U.S. Pat. No. 4,468,235 describes a method for extracting H2 contained in a mixture of fluids by bringing the mixture of fluids (liquid or gaseous) in contact with a membrane consisting of a titanium alloy containing ˜13% V, ˜11% Cr and ˜3% Al and carrying a metal selected from among Pd, Ni, Co, Fe, V, Nb or Ta, or an alloy containing one of these metals, on one of its faces.
Pd alloys, such as for example PdAg, PdCu, PdY, are considered to have good mechanical endurance to hydrogen and a permeability higher than that of palladium on its own (in particular Pd75Ag25). For example, U.S. Pat. No. 2,773,561 gives a comparison of the hydrogen permeability [expressed in cm3/s/cm2] of Pd and an alloy Pd75Ag25, which is summarized in the following table for membranes having a thickness of 25.4 μm.
450° C.550° C.Pressure (MPa)PdPdAgPdPdAg0.690.711.221.081.411.381.231.931.862.322.071.682.562.422.99
It is furthermore known that for an alloy Pd100-xCux in which x<30, the diffusion coefficient remains unchanged but the activation energy of the diffusion is about ⅓ that of Pd, and that the permeability Φ consequently increases, according to the equation
      Φ    =                  Φ        0            ⁢              ⅇ                  -                                    E              Φ                        T                                ,in which Φ0 is a constant (in mol.m−1.s−1.Pa−1/2), EΦ (in kelvin) is the activation energy of the diffusion, and T is the temperature (in K) (cf. “Diffusion of hydrogen in copper-palladium alloys”, J. Piper, J. Appl. Phys. Vol. 37, 715-721, 1966).
The hydrogen permeability of membranes consisting of Pd or Ni is described in particular in “Hydrogen permeability measurement through Pd, Ni and Fe membranes,” K. Yamakawa et al., J. Alloys and Compounds 321, 17-23, 2001.
Alloys based on palladium-silver are considered to exhibit efficient diffusion for hydrogen, in particular in “Investigation of Electromigration and Diffusion of Hydrogen in Palladium and PdAg Alloy”, R. Pietrzak et al., Defect and Diffusion Forum, vol 143-147, 951-956, 1997).
Membranes consisting of alloys of Pd (PdAg, PdY) on a ceramic support are selective for the separation of hydrogen from a gas mixture. [Cf. “Catalytic membrane reactors for tritium recovery from tritiated water in the ITER fuel cycle”, S. Tosti et al., Fusion Engineering and Design, Vol. 49-50, 953-958, 2000)].
U.S. Pat. No. 6,800,392 also describes the use of a membrane consisting of an alloy of Nb with from 5 to 25% of another metal selected from among Pd, Ru, Rh, Pt Au and Rh, the alloy membrane being obtained by colaminating films with different constituents. It is mentioned that the solubility of hydrogen in an alloy NbPd is about two times that of an alloy PdAg23.
Niobium has a very high permeability and is considered as the material most permeable to hydrogen in the study by REB Research & Consulting available at http://www.rebresearch.com/H2perm2.htm, from which FIG. 1 representing the permeability P in mol/mPa1/2s as a function of 1/T (K−1) is taken.
A permeability value of 3.2 10−7 mol.m−1.s−1.Pa−1/2 at 425° C. is furthermore put forward in Journal of Membrane Science, Vol. 85, 29-38, 1993. These properties, however, do not seem to be as beneficial at the temperatures at which supercapacitors operate (<100° C.). In particular, hydrogen forms a compound with niobium which is stable at low temperature, which mechanically weakens the niobium and limits the diffusion of hydrogen (cf. “Extractive Metallurgy of Niobium”, C. K. Gupta, CRC Press, 1994). Furthermore, niobium oxidizes very easily at room temperature. A barrier layer against the entry of hydrogen into the material is then formed on the surface. At room temperature, it is the phenomenon of an adsorption which most limits the diffusion of hydrogen through a niobium membrane. This is why the majority of authors publishing work on niobium report having worked with niobium covered with a very thin layer of palladium (thickness <1 μm): the palladium avoids the surface oxidation problems (its oxide is immediately reduced in the presence of hydrogen) and promotes the adsorption of hydrogen.
These reservations also apply to tantalum and vanadium. Although these materials seem beneficial at high temperature (>400° C.), at lower temperatures they have the same deficiencies as niobium: oxidation layer, weakening linked with the formation of stable metal-Hx compounds, low adsorption power. Here again, specialists generally recommend depositing a thin layer of palladium on the surface of the material for correct operation.
V—Ti—Ni alloys have a high hydrogen permeability, in particular the alloy V53Ti26Ni21 whose permeability is 1.0-3.7 10−9 mol.m−1.s−1.Pa−1/2: at 22° C., which is a value higher than that of palladium, namely 3.3-4.3 10−10. (Cf. “Hydrogen Permeability of Multiphase V—Ti—Ni Metallic Membranes”, Report under Contract No. DE-AC09-96SR18500 with the U.S. Department of Energy, T. M. Adams, J. Mickalonis).